1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to a disk drive employing a multi-stage pulse width modulated voice coil motor driver.
2. Description of the Prior Art
FIG. 1 shows a prior art disk drive comprising a disk 2 rotated about a center axis by a spindle motor (not shown). A head 4 attached to a distal end of an actuator arm 6 is actuated radially over the disk 2 by a voice coil motor (VCM) 8. The VCM 8 comprises a voice coil 10 which interacts with permanent magnets of a VCM yoke in order to rotate the actuator arm 6 about a pivot. The VCM 8 is typically driven in either a linear mode or in a pulse width modulated (PWM) mode. In addition, the motion of the VCM 8 may be controlled using a current feedback loop by sensing the amount of current flowing through the voice coil 10 which is proportional to the amount of torque applied to the actuator arm 6.
FIG. 1 also shows a VCM driver 12 comprising a conventional H-bridge driver for driving the voice coil 10 shown as a resistance Rvcm 14 and an inductance Lvcm 16. The H-bridge driver comprises a plurality of driver switches 18A–18D for selectively connecting the ends of the voice coil 10 to a supply voltage 20 or to ground 22 depending on the desired direction of rotation. A PWM signal 24 controls the switches 18A–18D in order to drive current through the voice coil 10 relative to the duty cycle of the PWM signal 24.
Driving the VCM 8 in a PWM mode reduces power dissipation in the driver switches 18A–18D as compared to driving the VCM 8 in a linear mode. However, PWM drivers are slow since an update can be made to the duty cycle only once for each PWM period. If the PWM frequency is increased to compensate for this delay, power dissipation increases due to the rise and fall time of the H-bridge driver, diminishing the benefit of the PWM mode. Additionally, edges of the PWM signal 24 propagate into the VCM wiring, inducing noise in the sensitive read channel circuitry.
There is, therefore, a need to drive a VCM of a disk drive in a PWM mode without requiring high clock speeds while providing adequate closed-loop frequency response.